1. Field
The subject matter of the present application relates generally to methods for calibrating an X-ray diagnostic system and apparatus for use in calibration methods.
2. Description of the Related Art
Medical interventions involving living subjects are increasingly performed using navigation assistance provided by a navigation support system. In some navigation support systems, a surgical instrument is guided by means of a position detection system relative to a tissue region of the subject undergoing treatment. Navigation assistance is of particular interest in body regions that cannot be visually inspected by the surgeon, such as when the instrument is inserted into the interior of the subject. For this purpose, the instrument, for example, a catheter, is guided in a virtual 3D volume generated by means of an imaging method prior to or during surgery. For example, an X-ray diagnostic machine may be used to generate a series of 2D projection images having a known projection geometry, and the 2D images may be used to generate a 3D volume data set. The 3D volume data set is transmitted to the navigation system, which is equipped with a position detection system for detecting positions of the markers. For high-precision navigation, the coordinate system of the position detection system can be aligned and/or oriented with the coordinate system of the 3D volume data set in a process commonly known as “registration.” In one example, the registration process uses a phantom, which includes X-ray positive markers and/or markers that are detectable by the position detection system in a fixed spatial relation to each other.
Some methods for improving the precision of a reconstructed 3D data set from the 2D X-ray projection images account for deviations of the projection geometry from the actual geometry of the imaging system. For example, mechanical flexure of the X-ray diagnostic machine may cause such deviations. Therefore, some X-ray diagnostic machines may be “calibrated” with special X-ray phantoms. In some cases, a calibration is performed only at certain times such as, for example, prior to shipment of the diagnostic machine from the factory, after a repair involving replacement of mechanical components of the machine, or prior to the start of an examination.
German Patent DE 102 02 091 A1 discloses a device and a method for determining a coordinate transformation using a phantom in which X-ray positive markers and markers that are detectable by a position detection system are disposed in fixed spatial relation to one another. During a scan to generate the 2D X-ray projection images, the coordinates of the X-ray positive markers are determined in the reconstructed 3D volume and transmitted to the position detection system and the navigation support system for calibration.
German Patent DE 100 47 382 C2 discloses an X-ray phantom that comprises markers that are detectable by a position detection system. Coordinates of the detected positions of the markers are measured in a coordinate system of the position detection system and in a coordinate system of the X-ray diagnostic system. The measured coordinates are used to calculate a coordinate transformation between these coordinate systems.
Many calibration methods using a position detection system suffer from disadvantages. For example, the positions of both the X-ray phantom and the X-ray beam receiver (or a part of the X-ray diagnostic system that is in a fixed spatial relation with the X-ray beam receiver according to a kinematic model) must be measured to calculate the coordinate transformation. Due to the inaccuracies of making at least two measurements, the coordinate transformation includes calculable error. For example, if the position detection system uses two cameras in fixed spatial relation to each other, the error in the coordinate system will increase as the angle between a point of the X-ray phantom or the X-ray diagnostic system and the two entrance pupils of the cameras gets smaller. In another example, if the position detection system includes an acquisition unit, the error in the coordinate system will increase as the distance between the acquisition unit and the X-ray phantom or the X-ray diagnostic system increases.